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<body>
<h1>第二章</h1>
<h2>1-Java的基本语法格式</h2>
<h2>2-Java中的注释</h2>
<h2>- 单行注释</h2>
<pre><code>//单行注释
</code></pre>

<ul>
<li>
<p>多行注释</p>
<p>/* 多行注释   */</p>
<p>不能嵌套</p>
</li>
<li>
<p>文档注释</p>
<p>说明文档</p>
<p>/**</p>
<ul>
<li>文档注释</li>
<li>
 */</li>
</ul>
</li>
</ul>
<h2>3-常量和变量</h2>
<p>起名字</p>
<p>标识符</p>
<p>任意顺序的大小写字母、数字、下划线（_）和美元符号（$）组成，但标识符不能以数字开头，也不能是Java中的关键字</p>
<p>例题：购物结算</p>
<pre><code>public static void main(String[] args) {
    // 变量存储可以变化的值
    // 1-声明商品名变量
    String productName1;
    String productName2;
    // 2-使用变量
    productName1 = &quot;面包&quot;;
    // 声明变量的同时赋值
    float productPrice1 = 7.5f;// 商品价格
    // Java中带小数点的数默认位double，使用float类型，数的后面+f
    float producrPrice2 = (float) 2.5;
    // 强制转换2.5为单精度数
    int number1 = 2;
    int number2 = 3;
    // 输出小票
    System.out.println(&quot;--------购物小票-------&quot;);
    System.out.println(&quot;商品名    价格      数量      总价&quot;);
    // System.out.println(&quot;面包          7.5   2    15&quot;);
    System.out.println(productName1 + &quot;    &quot; + productPrice1 + &quot;    &quot;
            + number1 + &quot;    &quot; + productPrice1 * number1);
    System.out.println(&quot;本              2.5   3    7.5&quot;);
    System.out.println(&quot;----------------------&quot;);
    System.out.println(&quot;总价格：22.5&quot;);
</code></pre>

<h2>4-输入数据</h2>
<p>输入操作，有封装好的方法，找到这个方法</p>
<p>Scanner</p>
<p>例题：		// 输入用户付款，计算找零并显示输出</p>
<pre><code>    float pay;// 付款金额
    float sum;// 商品总金额
    float repay;// 找零
    sum = 22.5f;// 计算结果=累加（商品单价*数量）
    Scanner in = new Scanner(System.in);// 默认键盘输入
    System.out.println(&quot;请输入付款金额&quot;);
    pay = in.nextFloat();
    System.out.println(&quot;付款：&quot; + pay);
</code></pre>

<p>问题：</p>
<ul>
<li>问题1-输入问题 实际输入的数据类型和要求输入的数据类型不一致 判断输入类型是否符合要求 1-符合要求--继续，2-不符合要求---重新输入</li>
<li>问题2-找零是负数。付款应该大于商品总金额 判断付款是否大于商品总金额 大于或等于---继续结算 小于---提示</li>
<li>问题3-每一个商品的名字单价和数量都需要定义一个变量保存 变量不好管理---数组 </li>
<li>问题4-通过商品的名字结算，输入编号（商品条码）</li>
</ul>

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